A transformer having a very low, predetermined leakage inductance is desirable for many power supply applications, such as, for example, synchronous rectifiers and radar power supplies. Low-profile magnetic devices can be constructed with "z-fold" magnetics; that is, the fabrication of magnetic devices using a pattern of conductive traces on flexible substrates which are z-folded (like an accordion) and inserted into a magnetic core, typically with a core post. An exemplary z-fold magnetic device is described in U.S. Pat. No. 5,126,715 of A. J. Yerman and W. A. Roshen, issued Jun. 30, 1992, assigned to the instant assignee, and incorporated by reference herein. The transformer of U.S. Pat. No. 5,126,715 includes a continuous, serpentine primary winding that is configured and z-folded to form a multi-pole, multi-layer winding having separate secondary winding layers interleaved with the primary winding. Other z-fold devices are described in U.S. Pat. Nos.: 4,943,793; 4,959,630; 5,017,902; 5,084,958; 5,134,770; 5,126,715; and 5,291,173; all of which are assigned to the instant assignee and incorporated by reference herein.
Although z-fold magnetics have resulted in much lower profile devices, such devices typically require at least two separate dielectric substrates with conductive film windings comprising the primary and secondary windings patterned thereon. Unfortunately, the distance between primary and secondary windings in such z-fold configurations is difficult to precisely determine and control, resulting in higher than desired leakage inductance values and/or tolerances.
Another type of magnetic device using dielectric substrates with conductive film windings patterned thereon is a barrel-wound type. Such windings are wound about a mandrel and inserted into a magnetic cup core with a cylindrical core post. Barrel-wound devices employing two separate dielectric substrates with conductive film windings share the same problems of higher and difficult to control leakage inductance as z-fold devices do. Another type of barrel-wound transformer uses a single dielectric substrate with primary and secondary windings patterned adjacent to each other, as described in Yerman U.S. Pat. No. 5,206,621, issued Apr. 27, 1993 and assigned to the instant assignee, which is incorporated by reference herein
Although the conductive windings of the types described hereinabove are fairly well interleaved, it is desirable to configure flexible conductive film windings to achieve a higher degree of interleaving and a low, controlled value of leakage inductance.